The present invention relates to a safety device which is carried by a lifeline or tether line and which will hold fast when weighted, but which is slidable along the lifeline or tether line when unweighted.
FIG. 1 illustrates a form of Prusik knot 10, also known as a triple sliding hitch or xe2x80x9cmonkey fist.xe2x80x9d Such a knot can be used in fall arrest safety systems which include a lifeline or tether line 12 having one end portion 14 adapted for connection to an anchoring device. For example, one end of the lifeline 12 can be formed with a sturdy eye 16 for receiving a carabiner 18 which, in turn, is secured to the eye of a roof anchor. The lifeline hangs from the anchor. The Prusik knot, when unweighted, can be slid along the lifeline as desired. A termination knot 20 can be provided to limit the extent of sliding movement of the Prusik knot along the lifeline.
In general, the Prusik knot can be formed of a single stretch of rope having several wraps 22 around the lifeline 12. The free end portions 24 of the rope are tucked through the longitudinally extending portion of a loop 26. In the embodiment illustrated in FIG. 1, the free ends of the rope are threaded through rings 28 of a thimble 30 which forms a central eye 32, and then are hand tied to form the end knot 33. A carabiner 34 is carried in the eye 32 and can be connected by a lanyard to a worker""s harness. In normal use, with little or no weight applied to the eye portion of the Prusik knot, the wraps 22 can be slid along the lifeline to a desired position. If substantial weight is applied to the eye, such as by way of the carabiner 34, the knot holds fast on the lifeline.
There are a myriad of regulations and specifications for fall arrest safety equipment. For the design shown in FIG. 1, the Prusik knot should contain at least six wraps 22. However, sometimes the knot will be unfastened by sliding the thimble 30 and rope ends 24 through the loop 26, resulting in fewer wraps 22 on the lifeline 12. The entire knot can be unfastened in this manner. Also, there can be concerns with respect to the type of connection from a user""s harness and/or lanyard to the eye 32. It is generally preferred that a large loop carabiner 34 be used, but sometimes a snap hook having a smaller opening will be connected to the thimble. In that case, a twisting force of the snap hook in the thimble eye may disengage the hook by damaging the swinging clasp of the snap hook. Also, in the construction of FIG. 1, some protection is formed for the inside portion of the rope loop around the thimble, but the exterior portion is exposed and may become frayed. Further, in fall arrest systems it is desirable for an integral component such as a the Prusik knot to be replaced if it has been subjected to the substantial force of a fall, but whether or not the knot has been subjected to such a force is not immediately ascertainable. Finally, it is possible for the hand tied knot 33 to become loosened over time, requiring frequent and careful inspection if the safety system utilizing the knot is to be reliable.
The present invention provides an improved rope grab device operating on the principles of a Prusik knot. In a preferred embodiment, a casing or cover is provided for the free end portions of the stretch of rope forming the knot. The cover can be formed of complimentary halves defining an internal cavity receiving the free end portions of the knot. One free end portion is much longer than the other and is looped in an internal channel to form a bight. This end portion can be secured to the other end portion, such as by a swage. The cover can completely encase the end portions of the knot so that they are not subjected to cutting or fraying, and can assist in reliably securing the free end portions of the knot without hand tying. In other aspects of the invention, the cover permits convenient and reliable interconnection with other components without compatibility problems such as can occur with certain snap hooks. The cover can be formed to provide a reliable indication of a strong force having been exerted on the safety device, and to lessen the likelihood of the knot unraveling.